If you want to get SLI running, instead of manually editing the xorg.conf you should copy/paste this command.
sudo nvidia-xconfig --sli=AFR
That will add the line
to your xorg.conf for Alternating Frame Rendering and you're less likely to make a mistake. Both "Yes" and "AFR" have worked for me but I would rather specify AFR because I have no idea what "YES" means to Nvidia. Alternately you can use SFR (Split Frame Rendering) or AA (Hellifino)